Scene contains too many lights for Corona to sample efficiently, a slower fallback sampler will be used. Current number of lights is X, but the light sampler can only handle 255 lights efficiently.

Note: this limitation only applies to pre-1.4 versions of Corona. The most reliable solution is updating Corona.

This message triggers when you exceed limit of 255 light groups in your scene. Each CoronaLight and each scene Mesh object with CoronaLightMTL is considered one light group, regardless of amount of polygons mesh object has. Once you exceed 255 light groups limit, Corona will switch to slower light sampler, that will often result in significantly more noise. 

256 lights, 2 minutes of rendering - excessive noise i visible
(click to enlarge)

255 lights, 2 minutes of rendering - noise level is clearly lower
(click to enlarge)


This limitation was removed in Corona 1.4 so the best solution is updating it.

To reduce amount of light groups in scene where large amount of lights is necessary, it is recommended to utilize mesh objects with CoronaLightMTL attached together by their proximity. If you attach several meshes with CoronaLightMTL to one, they will be considered one group. It is very important to note that you should only attach together CoronaLightMTL meshes that are close together and in the same part of the scene. For example if you have large city scene, then attach together light meshes in each building. If you have large house, attach together all lights in each room. But do not attach together lights that are far away from each other, or do not share any of the surface they illuminate. The goal here is not to have as few light groups as possible, but simply to not exceed 255 light group limits. So do not try to attach all scene mesh lights into one mesh. That will make rendering equally as slow as exceeding the limit, if not slower.

For some of the non important, dim glowing features in the scene that are high in count, you can replace CoronaLightMTL with CoronaMTL using self-illumination. Such meshes will then not count as light groups at all.

See: CoronaLightMtl or Self Illumination?